Aerostats include free balloons, airships, and moored balloons. An aerostat is a craft that can remain aloft through the use of buoyant lighter than air gases that impart lift to a vehicle. Aerostats use “aerostatic” lift, which is a buoyant force that does not require movement through the surrounding air mass. An aerostat's main structural component is its envelope, a lightweight skin containing a lifting gas to provide buoyancy, to which other components are attached. The vehicle has slightly less density than air and is thus kept aloft.
An aerostat can be used for numerous high-altitude activities. Payloads including instruments, communications equipment and the like can be coupled with or suspended from the aerostat. The payloads can be configured to conduct operations (e.g., observation, communication and the like) at the high altitudes, for instance an altitude of 20 miles. At such an altitude, a very large area can be surveyed, for example an area that encompasses a circle of a diameter over 600 miles. The equipment can function as a repeater, bouncing communication signals for many miles. By virtue of its high altitude, the vessel can be placed out of range of most small arms. While more sophisticated armaments can reach such a vessel, they would likely cost much more than the aerostat, perhaps 100 times as much, disincentivizing such an attack.
Aerostats contrast with aerodynes that primarily use aerodynamic lift which requires the movement of at least some part of the aircraft through the surrounding air mass. Airships, like other aircraft, generally rely on external control mechanisms (e.g., elevators, fins, rudders, etc.) to control the attitude of the airship and stabilize the airship in flight. Airships are generally subject to moments along two axes, which can be defined by two primary control vectors. The two primary control vectors are pitch (rotation about the lateral axis) and yaw (rotation about the vertical axis).
Conventional control systems rely on external control mechanisms to create aerodynamic forces causing the airship to pitch and/or yaw as desired, primarily to counteract external forces (e.g., a gust of wind or clouds) that would otherwise destabilize the airship in flight. Conventional control systems are also used to adjust the angle of attack for airships that depend on dynamic lift and reverse dynamic lift for takeoff and landing. These external control mechanisms, however, are inefficient because they add significant weight to the airship and their operation also generates drag, which slows the airship as it moves in the desired direction. These external control mechanisms generate drag even when they are not activated, because they tend to disrupt the aerodynamic shape of the airship. Additionally, these conventional control systems do not perform well in very slow or hovering flight because insufficient airflow is generated over the external control mechanisms.